Introduce replacement for ReactiveDataAccessStrategy

[mp911de]

[mp911de]

• getAllColumns and getIdentifierColumns -> RelationalPersistentEntity
• getOutboundRow -> UpdateMapper
• getRowMapper -> direct usage via EntityRowMapper and R2dbcConverter

[pmaedel]

I was able to use getAllColumns to create join assignments for a respective table alias:

inline fun <reified T : Any> ReactiveDataAccessStrategy.joinAssignments(tableAlias: String): String =
    getAllColumns(T::class.java).joinToString(", ") { column ->
        "$tableAlias.${column.reference.toLowerCase()} as ${tableAlias}_${column.reference.toLowerCase()}"
    }

Maybe it is a good idea to have this functionality integrated along the respective alias mapping requested in #448?